This invention relates to a system for the separation of phthalic anhydride from the reaction gas resulting from the catalytic oxidation of o-xylene with air.
Phthalic anhydride is conventionally formed by the gas phase oxidation of o-xylene with air in the presence of V.sub.2 O.sub.5 catalysts. The resultant reaction gas is passed to a separator where the phthalic anhydride is precipitated onto relatively cold pipes which are then heated to melt-off the precipitate. For the separation and melting off of the phthalic anhydride, the tubes are periodically cooled to a temperature in the range of 45.degree.-65.degree. C and heated to a temperature in the range of 150.degree.-250.degree. C, respectively. Such separators contain a large number of rows of tubes around which the reaction gas flows during the precipitation phase. During the continuous operation of these separators, it has now been found that the tube bundles last surrounded by the flowing reaction gas, i.e., the tubes on the gas outlet side of the separator, are gradually covered with deposits which, in contrast to the phthalic anhydride, can no longer be removed by melting with the use of the aforementioned heating temperatures. Because of this buildup, the flow resistance of the separator on the gas side increases by a multiple, and the separation efficiency which, in a separator without such deposits, is at above 99% of the physically separable matter, drops to 97%. This, in turn, results in a substantially increased load or even overload in the following waste gas purification stage. Still further, deposits result in an erosion and corrosion of the tubes and/or fins of the separator. Finally, in time, these difficult to melt deposits become so thick that the operation must be interrupted and the fouled tube bundles must be cleaned by hand. Thereafter, the tubes are subjected to repeated flushing with a suitable scrubbing, e.g. solution of sodium hydroxide. Such interruptions in the continuous process are highly undesirable since an additional separator must be kept in reserve for exchange purposes. Moreover, a complete elimination of the deposits is impossible, so that the next cleaning operation must be conducted after an even shorter operating period.